A lithium ion battery generally contains one or more electrochemical battery cells. Each battery cell typically includes a negative electrode, a positive electrode, and a separator situated between the electrodes. The separator facilitates operation of the electrochemical battery cell by providing a porous and electrically-insulative physical barrier between confronting faces of the two electrodes as is generally well understood in the art. A typical separator design, for instance, seeks a thin polymer structure that has a porosity sufficient to contain a liquid electrolyte that can communicate lithium ions while, at the same time, remains thermally, chemically, and mechanically stable enough to separate the confronting faces of the negative and positive electrodes over the course of many discharge/charge cell cycles so that a short-circuit is prevented. The most commonly used separators today are an extruded porous polyolefin sheet membrane—such as those made from polyethylene or polypropylene—or a laminate of several extruded porous polyolefin sheet membranes. Uniaxial or biaxial stretching is often relied upon during manufacture of the polyolefin sheet membranes(s) to promote the requisite porosity.
A conventional polyolefin sheet membrane, however, is potentially susceptible to certain performance declines when heated excessively. Exposure of the electrochemical battery cell to high temperatures associated with charging-phase heat generation, ambient atmospheric conditions, or some other source, for example, can cause the polyolefin sheet membrane to shrink, soften, and even melt. Such physical distortions of a polyolefin sheet membrane may ultimately permit the electrochemical battery cell to short-circuit through direct electrical contact between the confronting faces of the negative and positive electrodes. Battery thermal runaway is also a possibility if the electrodes come into direct electrical contact with one another to an appreciable extent. The tendency of an extruded and stretched polyolefin sheet membrane to lose some thermal stability for prolonged periods is a potential concern for some lithium ion battery applications.
A porous sheet membrane constructed from one of several types of engineering polymers that exhibit better thermal stability than a polyolefin could potentially enhance the temperature operating window of an electrochemical battery cell and, consequently, the lithium ion battery. But the techniques often used to make a porous polyolefin sheet membrane generally cannot transform the various types of engineering polymers into a sheet membrane that exhibits sufficient porosity across its thickness at reasonable costs. The stretching techniques used to make a polyolefin sheet membrane have also been shown to adversely affect the dimensional stability of a sheet membrane formed from certain engineering polymer materials when exposed elevated temperatures above 80° C. and, more noticeably, above 100° C. For example, when heated to such temperatures, a sheet membrane constructed from an engineering polymer may shrink in the direction that it was previously stretched.